1. Field of the Invention
This invention relates in general to vapor-cooled electrical apparatus and in particular to means for removal of moisture from such apparatus.
2. Description of the Prior Art
The combination of gas/vapor medium has proven to be a viable alternative to oil as a dielectric/cooling medium to be used in transformers, as well as other electric apparatus. The limiting factor retarding widespread use has been of an economic nature, i.e., oil is but a fraction of the cost of known vapor alternatives. With the increased efforts toward the development of non-flammable transformers that are environmentally acceptable, substantial research and development has highlighted the increased heat transfer capability of the vapor-cooled medium, while keeping the cost competitive with oil.
The dielectric strength as well as the lifetime of an electric insulation medium may be increased by removal of moisture from the medium. These phenomena were recognized by the transformer industry since its infancy and measures for moisture removal were taken since the beginning of the industry for the oil-filled units. Since the oil-filled units were generally vented to the atmosphere, these measures consisted of removal of the water vapor from the incoming air and/or removal of the absorbed water from the oil, usually by placing a desiccant material within the transformer, or removing the oil and reprocessing it.
New fire and explosion resistant transformers utilizing vapor-cooling rather than oil immersion must also have a method of moisture removal but now a new parameter is involved. Instead of being vented to the atmosphere as the oil immersion units, these new transformers are hermetically sealed systems since they utilize a vapor for the dielectric cooling medium.
In the prior art steps were taken to alleviate moisture problems in sealed systems by drying the components thoroughly before introducing the dielectric fluid and sealing the transformer. This technique did not suffice however, as predrying never removed all of the moisture present, and cellulose, a common material used for insulating the electrical windings, evolves water as a product of aging. This water promotes chemical reaction and further aging of the cellulose and further production of water. As mentioned previously, elimination of the water produced has been a continuing problem.
Known arrangements for removal of moisture from vapor-cooled apparatus include placing a desiccant material in the apparatus and placing a water vapor pervious membrane over an opening in the sealed container to transmit water vapor to the atmosphere while retaining all other materials inside the apparatus, this latter method employing a semi-sealed container. The problems involved are how to determine the necessary quantity of desiccant to remove moisture for the expected lifetime of the apparatus and how to limit the reverse flow of water vapor from the atmosphere through the membrane into the system.
Accordingly, it would be desirable to provide a simple and efficient means for separation, containment and subsequent elimination of water vapor from a hermetically sealed vapor-cooled apparatus.